Frequency division multiple access optical subcarriers

ABSTRACT

A network or system in which a hub or primary node may communicate with a plurality of leaf or secondary nodes. The hub node may operate or have a capacity greater than that of the leaf nodes. Accordingly, relatively inexpensive leaf nodes may be deployed to receive data carrying optical signals from, and supply data carrying optical signals to, the hub node. One or more connections may couple each leaf node to the hub node, whereby each connection may include one or more spans or segments of optical fibers, optical amplifiers, optical splitters/combiners, and optical add/drop multiplexer, for example. Optical subcarriers may be transmitted over such connections, each carrying a data stream. The subcarriers may be generated by a combination of a laser and a modulator, such that multiple lasers and modulators are not required, and costs may be reduced. As the bandwidth or capacity requirements of the leaf nodes change, the number of subcarriers, and thus the amount of data provided to each node, may be changed accordingly. Each subcarrier within a dedicated group of subcarriers may carry OAM or control channel information to a corresponding leaf node, and such information may be used by the leaf node to configure the leaf node to have a desired bandwidth or capacity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/813,151, filed Mar. 4, 2019, which is incorporatedherein by reference in its entirety.

BACKGROUND

In some optical communication systems, multiple optical signals, eachhaving a corresponding wavelength, and each being modulated to carry adifferent data stream, are multiplexed onto an optical fiber. In suchsystems, a laser and a modulator may be provided to generate eachoptical signal. Accordingly, in order to increase the capacity of suchsystems, additional lasers, modulators and associated circuitry areemployed. The cost associated with such systems may, therefore increaseas capacity is increased. Accordingly, there is a need for a morecost-effective network requiring fewer components, such as thecomponents described above.

Moreover, conventional optical communication systems may includehigh-speed circuitry and components to generate high-speed opticalsignals at a transmit end of the system. At a receive end, correspondinghigh-speed circuitry may be provided to detect the incoming data and toforward or distribute such data to lower capacity nodes. Accordingly,there is a further need to reduce costs by supplying high capacitysignals to less expensive lower capacity nodes without the need forintermediate high-speed circuitry and components at the receive end ofthe system.

In addition, in some conventional optical communication systems, datamay be transmitted as a series of frames, each of which includes apayload portion including customer or user data, and a header oroverhead portion including operation, administration, and maintenance(“OAM”) information associated with the system. With increasing networkcomplexity, the amount of such control or OAM information has increased,which may limit the amount of transmitted customer data. Therefore,there is also a need to transmit OAM information more efficiently sothat more customer data may be transmitted.

SUMMARY

Consistent the present disclosure, a network or system is provided inwhich a hub or primary node may communication with a plurality of remotenodes, such as leaf or secondary nodes. The hub node may operate or havea capacity that may be greater than that of the leaf nodes. Accordingly,relatively inexpensive leaf nodes may be deployed that receive datacarrying optical signals from, and supply data carrying optical signalsto, the hub node. One or more connections may couple each leaf node tothe hub node, whereby each connection may include one or more spans orsegments of optical fibers, optical amplifiers, opticalsplitters/combiners, and optical add/drop multiplexer, for example.Consistent with an aspect of the present disclosure, optical subcarriersmay be transmitted over such connections, each carrying a data stream.The subcarriers may be generated by a combination of a laser and amodulator, such that multiple lasers and modulators are not required,and costs may be reduced. In addition, the subcarriers may be employedusing multiple access techniques, such as frequency divisionmultiplexing access (FDMA), whereby a given group of subcarriers isdetected at a corresponding leaf node. Moreover, as the bandwidth orcapacity requirements of the leaf nodes change, the number ofsubcarriers, and thus the amount of data provided to each node may bechanged accordingly. In a further example, each subcarrier within adedicated group of subcarriers may carry OAM or control channelinformation to a corresponding leaf node, and such information may beused by the leaf node to configure the leaf node to have a desiredbandwidth or capacity.

The foregoing general description and the following detailed descriptionare exemplary and explanatory only and are not restrictive of theinvention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments and togetherwith the description, serve to explain the principles of the invention.

Other aspects, features and advantages will be apparent from thefollowing detailed description, the accompanying drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are block diagrams showing examples of networks consistentwith an aspect of the present disclosure;

FIG. 3 is a block diagram showing an example of a primary node andsecondary node, respectively, in accordance with an additional aspect ofthe present disclosure;

FIG. 4 is an example of a spectral plot showing optical subcarriersconsistent with an aspect of the present disclosure;

FIG. 5 is a block diagram showing an example of a network consistentwith a further aspect of the present disclosure;

FIG. 6 is an example of a spectral plot showing optical subcarriersconsistent with an additional aspect of the present disclosure;

FIG. 7 is a block diagram showing an example of a network consistentwith an additional aspect of the present disclosure;

FIG. 8 is an example of a spectral plot showing optical subcarriersconsistent with a further aspect of the present disclosure;

FIG. 9 shows an example of a primary node transmitter consistent withthe present disclosure;

FIG. 10a is a block diagram showing an example of a primary nodetransmitter digital signal processor (DSP) consistent with a furtheraspect of the present disclosure;

FIG. 10b illustrates a portion of a primary node transmitter DSP ingreater detail consistent with an aspect of the present disclosure;

FIG. 10c illustrates a portion of a primary node transmitter DSP ingreater detail consistent with another aspect of the present disclosure;

FIG. 11 shows an example of a secondary node receiver consistent withthe present disclosure;

FIG. 12 shows an example of a secondary node receiver DSP consistentwith the present disclosure;

FIG. 13a shows an example of a secondary node transmitter consistentwith an aspect of the present disclosure;

FIGS. 13b to 13j are examples of spectral plots of subcarriers outputfrom secondary nodes consistent with an additional aspect of the presentdisclosure;

FIG. 13k is an example of a spectral plot showing combined subcarrierssupplied to a primary node receiver;

FIG. 14 shows an example of a primary node receiver in accordance withan aspect of the present disclosure;

FIG. 15 shows an example of a guard band consistent with an additionalaspect of the present disclosure;

FIG. 16 shows an example of a mesh network configuration consistent witha further aspect of the present disclosure;

FIGS. 17, 18 a-18 c, and 19 show diagrams whereby subcarrier dataallocation may change over time consistent with an additional aspect ofthe present disclosure;

FIG. 20 is a flow chart of a method for determining time of flight anddata transmission timing from consistent with an aspect of the presentdisclosure;

FIG. 21 is a flow chart of a method for scheduling

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. In general, the same reference numbers will beused throughout the drawings to refer to the same or like parts. Theterms “hub,” “hub node,” and “primary node” are used interchangeablyherein. In addition, the terms “leaf,” “leaf node,” and “secondary node”are used interchangeably herein.

FIG. 1 illustrates an example of an aggregation network 100 consistentwith the present disclosure in which primary node 110 may communicatewith multiple secondary nodes 112-j to 112-m, which sometimes may bereferred to individually or collectively as secondary node(s) 112.Secondary nodes 112, in one example, are remote from primary node 110.Primary node 110 may transmit optical subcarriers, described in greaterdetail below, in a downstream direction onto an optical communicationpath 111, which, like each of optical communication paths 113-j to113-m, may include one or more segments of optical fiber, as well as oneor more optical amplifiers, reconfigurable add-drop multiplexers(ROADMs) or other optical fiber communication equipment. Splitter 114may be coupled to an end of optical communication path 111 to receivethe optical subcarriers and provide a power split portion of eachsubcarrier to a corresponding one of secondary nodes 112-j to 112-m viaa respective one of optical communication paths 113-j to 113-m.

As further shown in FIG. 1, primary node 110 has a data capacity toreceive n Gbit/s of data (e.g., a data stream) for transmission tosecondary node 112. Each secondary node 112 may receive and output to auser or customer a portion of the data input to primary node 110. Inthis example, secondary nodes 112-j, 112-k, 112-l, and 112-m output jGbit/s, k Gbit/s, l Gbit/s, and m Gbit/s of data (data streams),respectively, whereby the sum of the j, k, l, and m may equal n (wherej, k, l, m, and n are positive numbers).

FIG. 2 show transmission of additional subcarriers in an upstreamdirection from secondary nodes 112-j to 112-m to primary node 110. Asfurther shown in FIG. 2, each of secondary nodes 112-j to 112-m maytransmit a corresponding group of subcarriers or one subcarrier tooptical combiner 116 via a respective one of optical communication paths115-1 to 115-m. Optical combiner 116 may, in turn, combine the receivedoptical subcarriers from secondary nodes 112-j to 112-m onto opticalcommunication path 117. Optical communication paths 115-1 to 115-m and117 may have a similar construction as optical communication paths 111and 112-1 to 112-m.

As further shown in FIG. 2, each of secondary nodes 112-j to 112-mreceives a respective data stream having a corresponding data rate of jGbit/s, k Gbit/s, l Gbit/s, and m Gbit/s. At primary node 110, datacontained in these streams may be output such that the aggregate datasupplied by primary node 110 is n Gbit/s, such that, as noted above, nmay equal the sum of j, k, l, and m.

In another example, subcarriers may be transmitted in both an upstreamand downstream direction over the same optical communication path. Inparticular, selected subcarriers may be transmitted in the downstreamdirection from primary node 110 to secondary nodes 112, and othersubcarriers may be transmitted in the upstream direction from secondarynodes 112 to primary node 110.

In some implementations, network 100 may include additional primaryand/or secondary nodes and optical communication paths, fewer primaryand/or secondary nodes and optical communication paths, or may have aconfiguration different from that described above. For example, network100 may have a mesh configuration or a point-to-point configuration.

FIG. 3 illustrates primary node 110 in greater detail. Primary node 110may include a transmitter 202 that supplies a downstream modulatedoptical signal including subcarriers, and a receiver that 204 that mayreceive upstream subcarriers carrying data originating from thesecondary nodes, such as nodes 112-j to 112-m.

FIG. 3 further shows a block diagram of one of secondary nodes 112,which may include a receiver circuit 302 that receives one or moredownstream transmitted subcarriers, and a transmitter circuit 304 thattransmits one or more subcarriers in the upstream direction.

FIG. 4 illustrates an example of a transmission spectrum that canaccommodate twenty subcarriers (SC0 to SC19) that may be output fromprimary node transmitter 202. Each of subcarriers SC0 to SC19 has acorresponding one of frequencies f0 to f19. Subcarriers SC0 to SC19, inone example, are Nyquist subcarriers, which are a group of opticalsignals, each carrying data, wherein (i) the spectrum of each suchoptical signal within the group is sufficiently non-overlapping suchthat the optical signals remain distinguishable from each other in thefrequency domain, and (ii) such group of optical signals is generated bymodulation of light from a single laser. In general, each subcarrier mayhave an optical spectral bandwidth that is at least equal to the Nyquistfrequency, as determined by the baud rate of such subcarrier.

As noted above, each of secondary nodes 112 may include less expensivecomponents than the components included in primary node 110.Accordingly, the bandwidth or the data capacity of the secondary nodes112 may be less than that associated with primary node 110, such thatthe capacity associated with each secondary node 112 is less than thatof primary node 110.

For example, as further shown in FIG. 4, primary node 110 may have abandwidth BW-P, such that the data carried by each of subcarriers SC1 toSC20 may be processed, recovered, and output either from transmitter 202or received from receiver 204. On the other hand, each of secondarynodes 112-j to 112-m may have a respective one of bandwidths BW to BWm,such that each secondary node has a data processing capacity or iscapable of processing and outputting data carried by up to ninesubcarriers, in this example.

As noted above, in order to reduce network costs, less expensivecomponents, such as optical components and certain electricalcomponents, may, in certain examples, be capable of processing signalover of a limited frequency range or bandwidth that is less than therange of signal frequencies that may be accommodated by the optical andelectrical components in primary node 110. For example, electricalcomponents, such as digital-to-analog (DACs), analog-to-digitalconverters (ADCs), and digital signal processors (DSPs), and opticalcomponents, such as modulators, in secondary nodes 112 may have anassociated bandwidth that is less than corresponding, albeit moreexpensive, components in primary node 110.

Example bandwidths of each of secondary nodes 112 are further shown inFIG. 4. Namely, bandwidth BW associated with secondary node 112-jextends over or encompasses a range including frequencies f0 to f8 ofsubcarriers SC0 to SC8, respectively; bandwidth BWk associated withsecondary node 112-k extends over or encompasses a range includingfrequencies f5 to f13 of subcarriers SC5 to SC13, respectively;bandwidth BWl associated with secondary node 112-l extends over orencompasses a range including frequencies f10 to f18 of subcarriers SC10to SC18, respectively; and bandwidth BWm associated with secondary node112-m extends over or encompasses a range including frequencies f11 tof19 of subcarriers SC11 to SC19, respectively. On the other hand, thebandwidth of primary node 110, BW-P, encompasses the entire range offrequencies f0-f19 of subcarriers SC0 to SC19.

As further shown in FIG. 4, certain subcarriers have frequencies thatfall within multiple bandwidths. For example, subcarriers SC5 and SC6have frequencies that fall within bandwidth BW and bandwidth BWk. Thedata carried by such subcarriers may, therefore, be detected andselectively output from either secondary node 112-j or 112-k. Forexample, if a customer requires that more data be received and outputfrom secondary node 112-k and less data be output from secondary node112-j, nodes 112-j and 112-k may be controlled or dynamically configuredsuch that the data carried by subcarriers SC5 and SC6 may be assigned toand output from secondary node 112-k, but not secondary node 112-j.Accordingly, as described in greater detail below, the data output fromeach node may be adapted to customer requirements that vary over time.

As further shown in FIG. 4, certain subcarriers, such as subcarriersSC2, SC7, SC12, and SC17, may be designated or dedicated to carryinformation related to a parameter or characteristic associated with oneor more of secondary nodes 112. For example, such parameters maycorrespond to an amount of data, data rate, or capacity to be output byone or more secondary nodes. In particular, such subcarriers may carryinformation, for example, to configure or adjust the amount of data,capacity or data rate of data output from secondary nodes 112-j to112-m, respectively, as noted above. In a further example, each of thesesubcarriers may carry user or customer data (also referred to as clientdata) in addition to control information. In the example shown in FIG.4, only subcarriers SC2, SC7, SC12, and S17 are transmitted.

In a further example, subcarriers SC2, SC7, SC12, and SC17 are modulatedto carry control or OAM information and related data corresponding toparameters associated therewith, such as the capacity and status ofnodes 112. In an additional example, subcarrier SC2 is modulated carrysuch control and parameter information associated with node 112-j,subcarrier SC7 is modulated to carry such control and parameterinformation associated with node 112-k, subcarrier SC12 is modulated tocarry such control and parameter information associated with node 112-land subcarrier SC17 is modulated to carry such control and parameterinformation associated with node 112-m. In a further example, such SCsare modulated to carry information related to a parameter associated thetiming and scheduling of data transmission from the nodes 112 to primarynode 110, as described in detail below with respect to FIGS. 19-24.

FIG. 5 illustrates an example in which each of secondary nodes 112-j to112-m outputs equal amounts of user data originating from primary node110, and FIG. 6 illustrates how subcarriers SC0 to SC19 may be assignedor associated with each secondary node 112 to meet the data allocationshown in FIG. 5. In addition, in the example shown in FIGS. 5 and 6, theaggregate data input to primary node 110 may be 20 Gbit/s (n=20), andeach of secondary nodes 112 outputs an equal portion of such data, i.e.,5 Gbit/s, whereby j=k=l=m=5. Put another way, based on controlinformation received by node 112-j, for example, data assocai

Here, subcarriers SC0, SC1, SC3, and SC4 may be assigned to secondarynode 112-j (subcarrier SC2 further being assigned to and carryingcontrol channel information for secondary node 112-j); subcarriers SC5,SC6, SC8, and SC9 may assigned to secondary node 112-k (subcarrier SC7further being assigned to and carrying control channel information forsecondary node 112-k); subcarriers SC10, SC11, SC113, and SC14 mayassigned to secondary node 112-l (subcarrier SC12 further being assignedto and carrying control channel information for secondary node 112-l);and subcarriers SC15, SC16, SC18, and SC19 may assigned to secondarynode 112-m (subcarrier SC17 further being assigned to and carryingcontrol channel information for secondary node 112-m). Each subcarrier,in this example, may have an associated data rate of 1 Gbit/s.

Put another way, based on control information received by node 112-jcarried by subcarrier SC2, for example, a first portion of data input toDSP 902 described below and associated with subcarriers SC0, SC1, SC3,and SC4 is output from node 112-j, as well as any user data carried bysubcarrier SC2. Further, based on control information received by node112-k carried by subcarrier SC7, for example, a second portion of datainput to DSP 902 and associated with subcarriers SC5, SC6, SC8, and SC9is output from node 112-k, as well as any user data carried bysubcarrier SC7.

FIG. 7 illustrates another example in which the data allocation tosecondary nodes 112-j to 112-m may be changed based on, for example,changing capacity requirements. For example, at a point later in timethan that associated with the subcarrier allocation shown in FIG. 6,node 112-j, for example, may require more data than node 112-k. Here,control information carried by subcarriers SC2, SC7, SC12, and SC17.FIG. 8 illustrates how subcarriers SC0 to SC19 may be assigned orassociated with each secondary node 112 to satisfy the data allocationshown in FIG. 7. In this example, secondary node 112-j is allocated themaximum data capacity, i.e., data carried by seven subcarriers (SC0, SC1to SC3-SC6, and SC8) as well as any customer data that may be carried bysubcarrier SC2; secondary node 112-k is allocated ⅓ (i.e., 3/9) themaximum data capacity, i.e., data carried by three subcarriers(SC9-SC11) as well as any customer data that may be carried bysubcarrier SC7; secondary node 112-l is allocated 5/9 the maximum datacapacity, i.e., data carried by four subcarriers (SC13-SC16) as well asany customer data that may be carried by subcarrier SC12; and secondarynode 112-m is allocated a ⅓ (i.e., 3/9) the maximum data capacity, i.e.,data carried by two subcarriers (SC18 and SC19) as well as any customerdata that may be carried by subcarrier SC17.

That is, in the example described above in connection with FIG. 8, basedon additional (third) control information received by node 112-j carriedby subcarrier SC2, for example, a third portion of data input to DSP 902associated with subcarriers SC0, SC1, SC3-SC6, and SC8 is output fromnode 112-j, as well as any user data carried by subcarrier SC2. Also,based on (fourth) control information received by node 112-k carried bysubcarrier SC7, for example, a fourth portion of data input to DSP 902and associated with subcarriers SC9-SC11, is output from node 112-k, aswell as any user data carried by subcarrier SC7.

Thus, by communicating control information to secondary nodes the amountof data allocated, e.g., the data rate, for output from each secondarynode 112 may be controlled or varied over time based on such controlinformation. In the examples discussed above, the control informationmay identify the subcarriers associated with each node, and, therefore,the amount of data or data rate allocated to each node based on thenumber of subcarriers carrying data to be output from such node. Suchallocation, as noted above, may be changed dynamically, for example, inaccordance with varying data traffic requirements in network 100, andsuch allocation information may be carried, for example, by selected ordedicated subcarriers, as noted above, such as subcarriers SC2, SC7,SC12, and SC17.

Data allocation and subcarrier transmission are described next withreference to FIGS. 9 and 10 a-10 c.

FIG. 9 illustrates transmitter 202 of primary node 110 in greaterdetail. Transmitter 202 includes a plurality of circuits or switches SW,as well as a transmitter DSP (TX DSP) 902 and a D/A and optics block901. In this example, twenty switches (SW-0 to SW-19) are shown,although more or fewer switches may be provided than that shown in FIG.9. Each switch may have, in some instances, two inputs: the first inputmay receive user data, and the second input may receive controlinformation or signals (CNT). Each switch SW-0 to SW-19 can receive arespective one of control signals SWC-0 to SWC-19 output from controlcircuit 971, which may include a microprocessor, field programmable gatearray (FPGA), or other processor circuit. Based on the received controlsignal, each switch SW-0 to SW19 selectively outputs any one of the datastreams D-0 to D-19, or a control signal CNT-0 to CNT-19. Controlsignals CNT can be any combination of configuration bits for controland/or monitoring purposes. For example, control signals CNT may includeinstructions to one or more of secondary nodes 112 to change the dataoutput from such secondary nodes 112, such as by identifying thesubcarriers associated with such data. In another example, the controlsignals may include a series of known bits used in secondary nodes 112to “train” the receiver to detect and process such bits so that thereceiver can further process subsequent bits. In a further example, thecontrol channel CNT includes information that may be used by thepolarization mode dispersion (PMD) equalizer circuits 1125 discussedbelow to correct for errors resulting from polarization rotations of theX and Y components of one or more subcarriers (SC). In a furtherexample, control information CNT is used to restore or correct phasedifferences between laser transmit-side laser 908 and a local oscillatorlaser 1110 in each of the secondary nodes 112. Such detected phasedifferences may be referred to as cycle slips. In a further example,control information CNT may be used to recover, synchronize, or correcttiming differences between clocks provided in the primary (110) andsecondary nodes 112.

In another, example, one or more of switches SW may be omitted, andcontrol signals CNT may be supplied directly to DSP 902. Moreover, eachinput to DSP 902, such as the inputs to FEC encoders 1002 describedbelow (see FIG. 10a ), receives, in another example, a combination ofcontrol information described above as well as user data.

In a further example, control signal CNT includes information related tothe number of subcarriers that may be output from each of secondarynodes 112. Such selective transmission of subcarriers is described withreference to FIGS. 10a-10c . Although such description is in connectionwith primary node DSP 902, similar circuitry may be included in thesecondary node TX DSP 1302 (FIG. 13) to adjust or control the number ofsubcarriers output therefrom.

Based on the outputs of switches SW-0 to SW-19, DSP 902 may supply aplurality of outputs to D/A and optics block 901 includingdigital-to-analog conversion (DAC) circuits 904-1 to 904-4, whichconvert digital signal received from DSP 902 into corresponding analogsignals. D/A and optics block 901 also includes driver circuits 906-1 to906-2 that receive the analog signals from DACs 904-1 to 904-4 andadjust the voltages or other characteristics thereof to provide drivesignals to a corresponding one of modulators 910-1 to 910-4.

D/A and optics block 901 further includes modulators 910-1 to 910-4,each of which may be, for example, a Mach-Zehnder modulator (MZM) thatmodulates the phase and/or amplitude of the light output from laser 908.As further shown in FIG. 9, light output from laser 908, also includedin block 901, is split such that a first portion of the light issupplied to a first MZM pairing, including MZMs 910-1 and 910-2, and asecond portion of the light is supplied to a second MZM pairing,including MZMs 910-3 and 910-4. The first portion of the light is splitfurther into third and fourth portions, such that the third portion ismodulated by MZM 910-1 to provide an in-phase (I) component of an X (orTE) polarization component of a modulated optical signal, and the fourthportion is modulated by MZM 910-2 and fed to phase shifter 912-1 toshift the phase of such light by 90 degrees in order to provide aquadrature (Q) component of the X polarization component of themodulated optical signal. Similarly, the second portion of the light isfurther split into fifth and sixth portions, such that the fifth portionis modulated by MZM 910-3 to provide an I component of a Y (or TM)polarization component of the modulated optical signal, and the sixthportion is modulated by MZM 910-4 and fed to phase shifter 912-2 toshift the phase of such light by 90 degrees to provide a Q component ofthe Y polarization component of the modulated optical signal.

The optical outputs of MZMs 910-1 and 910-2 are combined to provide an Xpolarized optical signal including I and Q components and are fed to apolarization beam combiner (PBC) 914 provided in block 901. In addition,the outputs of MZMs 910-3 and 910-4 are combined to provide an opticalsignal that is fed to polarization rotator 913, further provided inblock 901, that rotates the polarization of such optical signal toprovide a modulated optical signal having a Y (or TM) polarization. TheY polarized modulated optical signal also is provided to PBC 914, whichcombines the X and Y polarized modulated optical signals to provide apolarization multiplexed (“dual-pol”) modulated optical signal ontooptical fiber 916, for example, which may be included as a segment ofoptical fiber in optical communication path 111.

The polarization multiplexed optical signal output from D/A and opticsblock 401 includes subcarriers SC0-SC19 noted above, such that eachsubcarrier has X and Y polarization components and I and Q components.Moreover, each subcarrier SC0 to SC19 may be associated with orcorresponds to a respective one of the outputs of switches SW-0 toSW-19. In one example, switches SW2, SW7, SW12, and SW17 may supplycontrol information carried by a respective one of control signalsCNT-2, CNT-7, CNT-12, and CNT-17 to DSP 902. Based on such controlsignals, DSP 902 provides outputs that result in optical subcarriersSC2, SC7, SC12, and SC17 carrying data indicative of the controlinformation carried by CNT-2, CNT-7, CNT-12, and CNT-17, respectively,as shown in FIGS. 4, 6, and 8. In addition, remaining subcarriers SC0,SC1, SC3 to SC6, SC8 to SC11, SC13 to SC16, and SC18 to SC20 carryinformation indicative of a respective one of data streams D-0, D-1,D-3-D-6, D-8 to D-11, D-13 to D-16, and D-18 to D-20 output from acorresponding one of switches SW0, SW1, SW3 to SW-6, SW-8 to SW11, SW13to SW16, and SW18 to SW20, as further shown in FIGS. 4, 6, and 8.

FIG. 10a shows an example of TX DSP 902 in greater detail. TX DSP 902may include FEC encoders 1002-0 to 1002-19, each of which may receive arespective one of a plurality of the outputs from switches SW0 to SW19.FEC encoders 1002-0 to 1002-19 carry out forward error correction codingon a corresponding one of the switch outputs, such as, by adding paritybits to the received data. FEC encoders 1002-0 to 1002-19 may alsoprovide timing skew between the subcarriers to correct for skew inducedby link between nodes 110 and 112-j to 112-m described above. Inaddition, FEC encoders 1002-0 to 1002-19 may interleave the receiveddata.

Each of FEC encoders 1002-0 to 1002-19 provides an output to acorresponding one of a plurality of bits-to-symbol circuits, 1004-0 to1004-19 (collectively referred to herein as “1004”). Each ofbits-to-symbol circuits 1004 may map the encoded bits to symbols on acomplex plane. For example, bits-to-symbol circuits 1004 may map fourbits to a symbol in a dual-polarization QPSK constellation. Each ofbits-to-symbol circuits 1004 provides first symbols, having the complexrepresentation XI+j*XQ, associated with a respective one of the switchoutputs, such as D-0, to DSP portion 1003. Data indicative of such firstsymbols is carried by the X polarization component of each subcarrierSC0-SC19.

Each of bits-to-symbol circuits 1004 further may provide second symbolshaving the complex representation YI+j*YQ, also associated with acorresponding output of switches SW0-SW19. Data indicative of suchsecond symbols, however, is carried by the Y polarization component ofeach of subcarriers SC-0 to SC-19.

Such mapping, as carried by about circuit 1004-0 to 1004-19 define, inone example, a particular modulation format for each subcarrier. Thatis, such circuit may define a mapping for all the optical subcarrierthat is indicative of a binary phase shift keying (BPSK) modulationformat, a quadrature phase shift keying (QPSK) modulation format, or anm-quadrature amplitude modulation (QAM, where m is a positive integer,e.g., 4, 8, 16, or 64) format. In another example, one or more of theoptical subcarriers may have a modulation format that is different thanthe modulation format of other optical subcarriers. That is, one of theoptical subcarriers have a QPSK modulation format and another opticalsubcarrier has a different modulation format, such as 8-QAM or 16-QAM.In another example, one of the optical subcarriers has an 8-QAMmodulation format and another optical subcarrier has a 16 QAM modulationformat. Accordingly, although all the optical subcarriers may carry dataat the same data and or baud rate, consistent with an aspect of thepresent disclosure one or more of the optical subcarriers may carry dataat a different data or baud rate than one or more of the other opticalsubcarriers. Moreover, modulation formats, baud rates and data rates maybe changed over time depending on capacity requirements, for example.Adjusting such parameters may be achieved, for example, by applyingappropriate signals to mappers 1004 based on control information or datadescribed herein and the communication of such data as further disclosedherein between hub and leaf nodes.

As further shown in FIG. 10a , each of the first symbols output fromeach of bits-to-symbol circuits 1004 is supplied to a respective one offirst overlap and save buffers 1005-0 to 1005-19 (collectively referredto herein as overlap and save buffers 1005) that may buffer 256 symbols,for example. Each of overlap and save buffers 1005 may receive 128 ofthe first symbols or another number of such symbols at a time from acorresponding one of bits to symbol circuits 1004. Thus, overlap andsave buffers 1005 may combine 128 new symbols from bits to symbolcircuits 1005, with the previous 128 symbols received from bits tosymbol circuits 1005.

Each overlap and save buffer 1005 supplies an output, which is in thetime domain, to a corresponding one of fast Fourier Transform (FFT)circuits 1006-0 to 1006-19 (collectively referred to as “FFTs 1006”). Inone example, the output includes 256 symbols or another number ofsymbols. Each of FFTs 1006 converts the received symbols to thefrequency domain using or based on, for example, a fast Fouriertransform. Each of FFTs 1006 may provide the frequency domain data tobins and switches blocks 1021-0 to 1021-19. As discussed in greaterdetail below, bins and switches blocks 1021 include, for example,memories or registers, also referred to as frequency bins (FB) orpoints, that store frequency components associated with each subcarrierSC.

Selected frequency bins FB are shown in FIG. 10b . Groups of suchfrequency bins FB are associated with give subcarriers. Accordingly, forexample, a first group of frequency bins, FB0-0 to FB0-n, is associatedwith SC0 and a second group of frequency bins FB19-0 to FB19-n with SC19(where n is a positive integer). As further shown in FIG. 10b , each offrequency bins FB is further coupled to a respective one of switches SW.For example, each of frequency bins FB0-0 to FB0-n is coupled to arespective one of switches SW0-0 to SW0-n, and each of FB19-0 to FB19-nis coupled to a respective one of switches or switch circuits SW19-0 toSW19-n.

Each switch SW selectively supplies either frequency domain data outputfrom one of FFT circuits 1006-0 to 1006-19 or a predetermined value,such as 0. In order to block or eliminate transmission of a particularsubcarrier, the switches SW associated with the group of frequency binsFB associated with that subcarrier are configured to supply the zerovalue to corresponding frequency bins. Accordingly, for example, inorder to block subcarrier SC0, switches SW0-0′ to SW0-n′ supply zero (0)values to a respective one of frequency bins FB0-0 to FB0-n. Furtherprocessing, as described below, of the zero (0) values by replicatorcomponents 1007 as well as other components and circuits in DSP 902result in drive signals supplied to modulators 910, such that subcarrierSC0 is omitted from the optical output from the modulators.

On the other hand, switches SW′ may be configured to supply the outputsof FFTs 1006, i.e., frequency domain data FD, to corresponding frequencybins FB. Further processing of the contents of frequency bins FB byreplicator components 1007 and other circuits in DSP 902 result in drivesignals supplied to modulators 910, whereby, based on such drivesignals, optical subcarriers are generated that correspond to thefrequency bin groupings associated with that subcarrier.

In the example discussed above, switches SW0-0′ to SW0-n′ supplyfrequency domain data FD0-0 to FD-n from FFT 1006-0 to a respective oneof switches SW0-0 to SW0-n. These switches, in turn, supply thefrequency domain data to a respective one of frequency bins FB0-0 toFB0-n for further processing, as described in greater detail below.

Each of replicator components or circuits 1007-0 to 1007-19 mayreplicate the contents of the frequency bins FB and store such contents(e.g., for T/2 based filtering of the subcarrier) in a respective one ofthe plurality of replicator components. Such replication may increasethe sample rate. In addition, replicator components or circuits 1007-0to 1007-19 may arrange or align the contents of the frequency bins tofall within the bandwidths associated with pulse shaped filter circuits1008-0 to 1008-19 described below.

Each of pulse shape filter circuits 1008-0 to 1008-19 may apply a pulseshaping filter to the data stored in the 512 frequency bins of arespective one of the plurality of replicator components or circuits1007-0 to 1007-19 to thereby provide a respective one of a plurality offiltered outputs, which are multiplexed and subject to an inverse FFT,as described below. Pulse shape filter circuits 1008-1 to 1008-19calculate the transitions between the symbols and the desired subcarrierspectrum so that the subcarriers can be packed together spectrally fortransmission, e.g., with a close frequency separation. Pulse shapefilter circuits 1008-0 to 1008-19 also may be used to introduce timingskew between the subcarriers to correct for timing skew induced by linksbetween nodes shown in FIG. 1, for example. Multiplexer component 1009,which may include a multiplexer circuit or memory, may receive thefiltered outputs from pulse shape filter circuits 1008-0 to 1008-19, andmultiplex or combine such outputs together to form an element vector.

Next, IFFT circuit or component 1010-1 may receive the element vectorand provide a corresponding time domain signal or data based on aninverse fast Fourier transform (IFFT). In one example, the time domainsignal may have a rate of 64 GSample/s. Take last buffer or memorycircuit 1011-1, for example, may select the last 1024 samples, oranother number of samples, from an output of IFFT component or circuit1010-1 and supply the samples to DACs 904-1 and 904-2 (see FIG. 9) at 64GSample/s, for example. As noted above, DAC 904-1 is associated with thein-phase (I) component of the X pol signal, and DAC 904-2 is associatedwith the quadrature (Q) component of the Y pol signal. Accordingly,consistent with the complex representation XI+jXQ, DAC 904-1 receivesvalues associated with XI and DAC 904-2 receives values associated withjXQ. As indicated by FIG. 9, based on these inputs, DACs 904-1 and 904-2provide analog outputs to MZMD 906-1 and MZMD 906-2, respectively, asdiscussed above.

As further shown in FIG. 10a , each of bits-to-symbol circuits 1004-0 to1004-19 outputs a corresponding one of symbols indicative of datacarried by the Y polarization component of the polarization multiplexedmodulated optical signal output on fiber 916. As further noted above,these symbols may have the complex representation YI+j*YQ. Each suchsymbol may be processed by a respective one of overlap and save buffers1015-0 to 1015-19, a respective one of FFT circuits 1016-0 to 1016-19, arespective one of replicator components or circuits 1017-0 to 517-19,pulse shape filter circuits 1018-0 to 1018-19, multiplexer or memory1019, IFFT 1010-2, and take last buffer or memory circuit 1011-2, toprovide processed symbols having the representation YI+j*YQ in a mannersimilar to or the same as that discussed above in generating processedsymbols XI+j*XQ output from take last circuit 1011-1. In addition,symbol components YI and YQ are provided to DACs 904-3 and 904-4 (FIG.9), respectively. Based on these inputs, DACs 904-3 and 904-4 provideanalog outputs to MZMD 906-3 and MZMD 906-4, respectively, as discussedabove.

While FIG. 10a shows DSP 902 as including a particular number andarrangement of functional components, in some implementations, DSP 902may include additional functional components, fewer functionalcomponents, different functional components, or differently arrangedfunctional components. In addition, typically the number of overlap andsave buffers, FFTs, replicator circuits, and pulse shape filtersassociated with the X component may be equal to the number of switchoutputs, and the number of such circuits associated with the Y componentmay also be equal to the number of switch outputs. However, in otherexamples, the number of switch outputs may be different from the numberof these circuits.

As noted above, based on the outputs of MZMDs 906-1 to 906-4, aplurality of optical subcarriers SC0 to SC19 may be output onto opticalfiber 916 (FIG. 9), which is coupled to the primary node 110.

Consistent with an aspect of the present disclosure, the number ofsubcarriers transmitted from primary node 110 to secondary nodes 112 mayvary over time based, for example, on capacity requirements at theprimary node and the secondary nodes. For example, if less downstreamcapacity is required initially at one or more of the secondary nodes,transmitter 202 in primary node 110 may be configured to output feweroptical subcarriers. On the other hand, if further capacity is requiredlater, transmitter 202 may provide more optical subcarriers.

In addition, if based on changing capacity requirements, a particularsecondary node 112 needs to be adjusted, for example, the outputcapacity of such secondary node may be increased or decreased by, in acorresponding manner, increasing or decreasing the number of opticalsubcarriers output from the secondary node.

As noted above, by storing and subsequently processing zeros (0s) orother predetermined values in frequency bin FB groupings associated witha given subcarrier SC, that subcarrier may be removed or eliminated. Toadd or reinstate such subcarrier, frequency domain data output from theFFTs 1006 may be stored in frequency bins FB and subsequently processedto provide the corresponding subcarrier. Thus, subcarriers may beselectively added or removed from the optical outputs of primary nodetransmitter 202 and secondary node transmitter 304, such that the numberof subcarriers output from such transmitters may be varied, as desired.

In the above example, zeros (0s) or other predetermined values arestored in selected frequency bins FBs to prevent transmission of aparticular subcarrier SC. Such zeroes or values may, instead, beprovided, for example, in a manner similar to that described above, atthe outputs of corresponding replicator components 1007 or stored incorresponding locations in memory or multiplexer 1009. Alternatively,the zeroes or values noted above may be provided, for example, in amanner similar to that described above, at corresponding outputs ofpulse shape filters 1008.

In a further example, a corresponding one of pulse shape filters 1008-1to 1008-19 may selectively generate zeroes or predetermined values that,when further processed, also cause one or more subcarriers SC to beomitted from the output of either primary node transmitter 202 orsecondary node transmitter 304. In particular, as shown in FIG. 10c ,pulse shape filters 1008-0 to 1008-19 are shown as including groups ofmultiplier circuits M0-0 to M0-n M19-0 to M19-n (also individually orcollectively referred to as M). Each multiplier circuit M constitutespart of a corresponding butterfly filter. In addition, each multipliercircuit grouping is associated with a corresponding one of subcarriersSC.

Each multiplier circuit M receives a corresponding one of outputgroupings RD0-0 to RD0-n . . . RD19-0 to RD19-n from replicatorcomponents 1007. In order to remove or eliminate one of subcarriers SC,multiplier circuits M receiving the outputs within a particular groupingassociated with that subcarrier multiply such outputs by zero (0), suchthat each multiplier M within that group generates a product equal tozero (0). The zero products then are subject to further processingsimilar to that described above to provide drive signals to modulators910 that result in a corresponding subcarrier SC being omitted from theoutput of the transmitter (either transmitter 202 or 304).

On the other hand, in order to provide a subcarrier SC, each of themultiplier circuits M within a particular groping may multiply acorresponding one of replicator outputs RD by a respective one ofcoefficients C0-0 to C0-n . . . C19-0 to C19-n, which results in atleast some non-zero products being output. Based on the products outputfrom the corresponding multiplier grouping, drive signals are providedto modulators 910 to output the desired subcarrier SC from thetransmitter (either transmitter 202 or 304).

Accordingly, for example, in order to block or eliminate subcarrier SC0,each of multiplier circuits M0-0 to M0-n (associated with subcarrierSC0) multiplies a respective one of replicator outputs RD0-0 to RD0-n byzero (0). Each such multiplier circuit, therefore, provides a productequal to zero, which is further processed, as noted above, such thatresulting drive signals cause modulators 910 to provide an opticaloutput without SC0. In order to reinstate SC0, multiplier circuits M0-0to M0-n multiply a corresponding one of appropriate coefficients C0-0 toC0-n by a respective one of replicator outputs RD0-0 to RD0-n to provideproducts, at least some of which are non-zero. Based on these products,as noted above, modulator drive signals are generated that result insubcarrier SC0 being output.

The above examples are described in connection with generating orremoving the X component of a subcarrier SC. The processes and circuitrydescribed above is employed or included in DSP 902 and optical circuitryused to generate the Y component of the subcarrier to be blocked. Forexample, switches and bins circuit blocks 1022-0 to 1022-19, have asimilar structure and operate in a similar manner as switches and binscircuit blocks 1021 described above to provide zeroes or frequencydomain data as the case may be to selectively block the Y component ofone or more subcarriers SC. Alternatively, multiplier circuits, likethose described above in connection with FIG. 10c may be provided tosupply zero products output from selected pulse shape filters 1018 inorder to block the Y component of a particular subcarrier or, ifnon-zero coefficients are provided to the multiplier circuits instead,generate the subcarrier.

Thus, the above examples illustrate mechanisms by which subcarriers SCmay be selectively blocked from or added to the output of transmitter202. Since, as discussed below, DSPs and optical circuitry provided insecondary node transmitters 304 are similar to that of primary nodetransmitter 202, the processes and circuitry described above isprovided, for example, in the secondary node transmitters 304 toselectively add and remove subcarriers SC′ from the outputs of thesecondary node transmitters, as described in connection with FIGS.13b-13k . Moreover, consistent with the present disclosure, thecircuitry described above in connection with FIGS. 10b and/or 10 c maybeconfigured so that a first number of optical subcarriers are output fromthe transmitter (in either the primary node 110 or the secondary node112) during a first period of time based on initial capacityrequirements. Later, during a second period of time, a second number ofoptical subcarriers can be output from the hub and/or leaf transmittersbased on capacity requirements different than the first capacityrequirements.

Optical subcarriers SC0 to SC19 may be provided to secondary nodes 112in FIG. 1. An example of receiver circuit 302 in one of secondary nodes112 will be described next with reference to FIG. 11 a.

As shown in FIG. 11a , optical receiver 302 may include an Rx optics andND block 1100, which, in conjunction with DSP 1150, may carry outcoherent detection. Block 1100 may include a polarization splitter (PBS)1105 with first (1105-1) and second (1105-2) outputs), a localoscillator (LO) laser 1110, 90 degree optical hybrids or mixers 1120-1and 1120-2 (referred to generally as hybrid mixers 1120 and individuallyas hybrid mixer 1120), detectors 1130-1 and 1130-2 (referred togenerally as detectors 1130 and individually as detector 1130, eachincluding either a single photodiode or balanced photodiode), ACcoupling capacitors 1132-1 and 1132-2, transimpedanceamplifiers/automatic gain control circuits TIA/AGC 1134-1 and 1134-2,ADCs 1140-1 and 1140-2 (referred to generally as ADCs 1140 andindividually as ADC 1140).

Polarization beam splitter (PBS) 1105 may include a polarizationsplitter that receives an input polarization multiplexed optical signalincluding optical subcarriers SC0 to SC19 supplied by optical fiber link1101, which may be, for example, an optical fiber segment as part of oneof optical communication paths 113-k to 113-m noted above. PBS 1105 maysplit the incoming optical signal into the two X and Y orthogonalpolarization components. The Y component may be supplied to apolarization rotator 1106 that rotates the polarization of the Ycomponent to have the X polarization. Hybrid mixers 1120 may combine theX and rotated Y polarization components with light from local oscillatorlaser 1110, which, in one example, is a tunable laser. For example,hybrid mixer 1120-1 may combine a first polarization signal (e.g., thecomponent of the incoming optical signal having a first or X (TE)polarization output from a first PBS port with light from localoscillator 1110, and hybrid mixer 1120-2 may combine the rotatedpolarization signal (e.g., the component of the incoming optical signalhaving a second or Y (TM) polarization output from a second PBS port)with the light from local oscillator 1110. In one example, polarizationrotator 1190 may be provided at the PBS output to rotate Y componentpolarization to have the X polarization.

Detectors 1130 may detect mixing products output from the opticalhybrids, to form corresponding voltage signals, which are subject to ACcoupling by capacitors 1132-1 and 1132-1, as well as amplification andgain control by TIA/AGCs 1134-1 and 1134-2. The outputs of TIA/AGCs1134-1 and 1134-2 and ADCs 1140 may convert the voltage signals todigital samples. For example, two detectors (e.g., photodiodes) 1130-1may detect the X polarization signals to form the corresponding voltagesignals, and a corresponding two ADCs 1140-1 may convert the voltagesignals to digital samples for the first polarization signals afteramplification, gain control and AC coupling. Similarly, two detectors1130-2 may detect the rotated Y polarization signals to form thecorresponding voltage signals, and a corresponding two ADCs 1140-2 mayconvert the voltage signals to digital samples for the secondpolarization signals after amplification, gain control and AC coupling.RX DSP 1150 may process the digital samples associated with the X and Ypolarization components to output data associated with one or moresubcarriers within a group of subcarriers SC0 to SC19 encompassed by thebandwidth (one of bandwidths BW, BWk, BWl, and BWm) associated with thesecondary node housing the particular DSP 1150. For example, as shown inFIG. 6, subcarriers SC0 to SC8 are within bandwidth BSj, and suchsubcarriers may be processed by the receiver in secondary node 112-j. Asnoted above, however, subcarriers SC5 to SC13 are within bandwidth BWkmay be processed by the receiver in secondary node 112-k. That is,bandwidths BWj and BWk overlap, such that subcarriers within theoverlapped portions of these bandwidths, namely, subcarriers SC5 to SC8,will be processed by the receivers in both secondary node 112-j and112-k. If the data associated with these subcarriers is intended to beoutput from secondary node 112-k, switch circuits, for example, may beprovided to selectively output such day at node 112-k but not from node112-j, as discussed in greater detail below.

While FIG. 11a shows optical receiver 302 as including a particularnumber and arrangement of components, in some implementations, opticalreceiver 302 may include additional components, fewer components,different components, or differently arranged components. The number ofdetectors 1130 and/or ADCs 1140 may be selected to implement an opticalreceiver 302 that is capable of receiving a polarization multiplexedsignal. In some instances, one of the components illustrated in FIG. 11amay carry out a function described herein as being carry out by anotherone of the components illustrated in FIG. 11 a.

Consistent with the present disclosure, in order to select a particularsubcarrier or group of subcarriers at a secondary node 112, localoscillator 1110 may be tuned to output light having a wavelength orfrequency relatively close to the selected subcarrier wavelength(s) tothereby cause a beating between the local oscillator light and theselected subcarrier(s). Such beating will either not occur or will besignificantly attenuated for the other non-selected subcarriers so thatdata carried by the selected subcarrier(s) is detected and processed byDSP 1150.

As noted above, each secondary node 112 may have a smaller bandwidththan the bandwidth associated with primary node 110. The subcarriersencompassed by each secondary node may be determined by the frequency ofthe local oscillator laser 1110 in the secondary node receiver 302. Forexample, as shown in FIG. 11b , bandwidth BW associated with secondarynode 112-j may be centered about local oscillator frequency fLOj,bandwidth BWk associated with secondary node 112-k may be centered aboutlocal oscillator frequency fLOk, bandwidth BWl associated with secondarynode 112-l may be centered about local oscillator frequency fLOl, andbandwidth BWm associated with secondary node 112-m may be centered aboutlocal oscillator frequency fLOm. Accordingly, each bandwidth BW to BWmmay shift depending on the frequency of each secondary node localoscillator laser 1110. Tuning the local oscillator frequency, forexample, by changing the temperature of the local oscillator laser 1110may result in corresponding shifts in the bandwidth to encompass adifferent group of subcarriers than were detected prior to suchbandwidth shift. The temperature of the local oscillator laser 1110 maybe controlled with a thin film heater, for example, provided adjacentthe local oscillator laser or to portions of the local oscillator lasersuch as the mirror sections. Alternatively, the local oscillator lasermay be frequency tuned by controlling the current supplied to the laser.The local oscillator laser 1110 may be a semiconductor laser, such as adistributed feedback laser or a distributed Bragg reflector laser.

The maximum bandwidth or number of subcarriers that may be received,detected, and processed by a secondary node receiver 302, however, maybe restricted based on hardware limitations of the various circuitcomponents in receiver 302, as noted above, and, therefore, may befixed, in this example. Accordingly, as noted above, the bandwidthassociated with each secondary node 112 may be less than bandwidth BW-Passociated with primary node 110. Further, consistent with the presentdisclosure, the number of secondary nodes may be greater than the numberof subcarriers output from primary node 110. In addition, the number ofupstream subcarriers received by primary node 110 may be equal to thenumber of subcarriers transmitted by primary node 110 in the upstreamdirection. Alternatively, the number of subcarriers transmitted in theupstream direction collectively by secondary nodes 112 may less than orgreater than the number of downstream subcarriers output from theprimary node. Further, in another example, one or more of secondarynodes 112 may output a singe subcarrier.

As shown in FIG. 11b and discussed above, the bandwidths associated withsecondary nodes 112 may overlap, such that, as further noted above,certain subcarriers SC may be detected by multiple secondary nodes 112.If the data associated with such subcarriers SC is intended for one ofthose secondary nodes, but not the other, switch circuitry, as notedabove, may be provided in the secondary nodes to output the dataselectively at the intended secondary node but not the others.

For example, as further shown in FIG. 11a , switches or circuits SW-0 toSW-8 may be provided at the output of DSP 1150 to selectively output thedata detected from the received subcarriers based on a respective one ofcontrol signals CNT-0 to CNT-8 output from control circuit 1171, which,like control circuit 971 noted above may include a microprocessor, FPGA,or other processor circuit. Control signals may designate the output ofeach respective switch. Accordingly, for example, if data carried bypredetermined subcarriers is intended to be output at a particularsecondary node 112, switches SW at that secondary node may beconfigured, based on the received control signals CNT, to supply thedesired data, but block data not intended for that node.

FIG. 12 illustrates exemplary components of receiver digital signalprocessor (DSP) 1150. As noted above, analog-to-digital (ND) circuits1140-1 and 1140-2 (FIG. 11a ) output digital samples corresponding tothe analog inputs supplied thereto. In one example, the samples may besupplied by each A/D circuit at a rate of 64 GSamples/s. The digitalsamples correspond to symbols carried by the X polarization of theoptical subcarriers and may be represented by the complex number XI+jXQ.The digital samples may be provided to overlap and save buffer 1205-1,as shown in FIG. 12. FFT component or circuit 1210-1 may receive the2048 vector elements, for example, from the overlap and save buffer1205-1 and convert the vector elements to the frequency domain using,for example, a fast Fourier transform (FFT). The FFT component 1210-1may convert the 2048 vector elements to 2048 frequency components, eachof which may be stored in a register or “bin” or other memory, as aresult of carrying out the FFT.

The frequency components then may be demultiplexed by demultiplexer1211-1, and groups of such components may be supplied to a respectiveone of chromatic dispersion equalizer circuits CDEQ 1212-1-0 to1212-1-8, each of which may include a finite impulse response (FIR)filter that corrects, offsets or reduces the effects of, or errorsassociated with, chromatic dispersion of the transmitted opticalsubcarriers. Each of CDEQ circuits 1212-1-0 to 1212-1-8 supplies anoutput to a corresponding polarization mode dispersion (PMD) equalizercircuit 1225-0 to 1225-8 (which individually or collectively may bereferred to as 1225).

Digital samples output from ND circuits 640-2 associated with Ypolarization components of subcarrier SC1 may be processed in a similarmanner to that of digital samples output from ND circuits 1240-1 andassociated with the X polarization component of each subcarrier. Namely,overlap and save buffer 1205-2, FFT 1210-2, demultiplexer 1211-2, andCDEQ circuits 1212-2-0 to 1212-2-8 may have a similar structure andoperate in a similar fashion as buffer 1205-1, FFT 1210-1, demultiplexer122-1, and CDEQ circuits 1212-1-0 to 1212-1-8, respectively. Forexample, each of CDEQ circuits 1212-2-0 to 1212-8 may include an FIRfilter that corrects, offsets, or reduces the effects of, or errorsassociated with, chromatic dispersion of the transmitted opticalsubcarriers. In addition, each of CDEQ circuits 1212-2-0 to 1212-2-8provide an output to a corresponding one of PMDEQ 1225-0 to 1225-8.

As further shown in FIG. 12, the output of one of the CDEQ circuits,such as CDEQ 1212-1-0 ma be supplied to clock phase detector circuit1213 to determine a clock phase or clock timing associated with thereceived subcarriers. Such phase or timing information or data may besupplied to ADCs 1140-1 and 1140-2 to adjust or control the timing ofthe digital samples output from ADCs 1140-1 and 1140-2.

Each of PMDEQ circuits 1225 may include another FIR filter thatcorrects, offsets or reduces the effects of, or errors associated with,PMD of the transmitted optical subcarriers. Each of PMDEQ circuits 1225may supply a first output to a respective one of IFFT components orcircuits 1230-0-1 to 1230-8-1 and a second output to a respective one ofIFFT components or circuits 1230-0-2 to 1230-8-2, each of which mayconvert a 256-element vector, in this example, back to the time domainas 256 samples in accordance with, for example, an inverse fast Fouriertransform (IFFT).

Time domain signals or data output from IFFT 1230-0-1 to 1230-8-1 aresupplied to a corresponding one of XpoI carrier phase correctioncircuits 1240-1-1 to 1240-8-1, which may apply carrier recoverytechniques to compensate for X polarization transmitter (e.g., laser908) and receiver (e.g., local oscillator laser 1110) linewidths. Insome implementations, each carrier phase correction circuit 1240-0-1 to1240-8-1 may compensate or correct for frequency and/or phasedifferences between the X polarization of the transmit signal and the Xpolarization of light from the local oscillator 1100 based on an outputof XpoI carrier recovery circuit 1240-0-1, which performs carrierrecovery in connection with one of the subcarrier based on the outputsof IFFT 1230-01. After such X polarization carrier phase correction, thedata associated with the X polarization component may be represented assymbols having the complex representation xi+j*xq in a constellation,such as a QPSK constellation or a constellation associated with anothermodulation formation, such as an m-quadrature amplitude modulation(QAM), m being an integer. In some implementations, the taps of the FIRfilter included in one or more of PMDEQ circuits 1225 may be updatedbased on the output of at least one of carrier phase correction circuits1240-0-1 to 1240-8-01.

In a similar manner, time domain signals or data output from IFFT1230-0-2 to 1230-8-2 are supplied to a corresponding one of YpoI carrierphase correction circuits 1240-0-2 to 1240-8-2, which may compensate orcorrect for Y polarization transmitter (e.g., laser 908) and receiver(e.g., local oscillator laser 1110) linewidths. In some implementations,each carrier phase correction circuit 1240-0-2 to 1240-8-2 also maycorrect or compensate for frequency and/or phase differences between theY polarization of the transmit signal and the Y polarization of lightfrom the local oscillator 1110. After such Y polarization carrier phasecorrection, the data associated with the Y polarization component may berepresented as symbols having the complex representation yi+j*yq in aconstellation, such as a QPSK constellation or a constellationassociated with another modulation formation, such as an m-quadratureamplitude modulation (QAM), m being an integer. In some implementations,the output of one of circuits 1240-0-2 to 1240-8-2 may be used to updatethe taps of the FIR filter included in one or more of PMDEQ circuits1225 instead of, or in addition to, the output of at least one of thecarrier recovery circuits 1240-0-1 to 1240-8-1.

As further shown in FIG. 12, the output of carrier recovery circuits,e.g., carrier recovery circuit 1240-0-1, also may be supplied to carrierphase correction circuits 1240-1-1 to 1240-8-1 and 1240-0-2 to 1240-8-2,whereby the phase correction circuits may determine or calculate acorrected carrier phase associated with each of the received subcarriersbased on one of the recovered carriers, instead of providing multiplecarrier recovery circuits, each of which is associated with acorresponding subcarrier. The equalizer, carrier recovery, and clockrecovery may be further enhanced by utilizing the known (training) bitsthat may be included in control signals CNT, for example by providing anabsolute phase reference between the transmitted and local oscillatorlasers.

Each of the symbols-to-bits circuits or components 1245-0-1 to 1245-8-1may receive the symbols output from a corresponding one of circuits1240-0-1 to 1240-8-1 and map the symbols back to bits. For example, eachof the symbol-to-bits components 1245-0-1 to 1245-8-1 may map one Xpolarization symbol, in a QPSK or m-QAM constellation, to Z bits, whereZ is an integer. For dual-polarization QPSK modulated subcarriers, Z isfour. Bits output from each of component 1245-0-1 to 1245-8-1 areprovided to a corresponding one of FEC decoder circuits 1260-0 to1260-8.

Y polarization symbols are output form a respective one of circuits1240-0-2 to 1240-8-2, each of which has the complex representationyi+j*yq associated with data carried by the Y polarization component.Each Y polarization, like the X polarization symbols noted above, may beprovided to a corresponding one of bit-to-symbol circuits or components1245-0-2 to 1245-8-2, each of which has a similar structure and operatesin a similar manner as symbols-to-bits component 1245-0-1 to 1245-8-1.Each of circuits 1245-0-2 to 1245-8-2 may provide an output to acorresponding one of FEC decoder circuits 1260-0 to 1260-8.

Each of FEC decoder circuits 1260 may remove errors in the outputs ofsymbol-to-bit circuits 1245 using, for example, forward errorcorrection. Such error corrected bits, which may include user data foroutput from secondary node 112, may be supplied to a corresponding oneof switch circuits SW-0 to SW-8. As noted above, switch circuits SW-0 toSW-8 in each secondary node 112 may selectively supply or block databased on whether such data is intended to be output from the secondarynode. In addition, if one of the received subcarriers' controlinformation (CNT), such as information identifying switches SW thatoutput data and other switches SW that block data, the controlinformation may be output from one of the switches and, based on suchcontrol information, control circuit 1171 in the secondary nodes togenerate the control signals CNT.

Consistent with another aspect of the present disclosure, data may beblocked from output from DSP 1150 without the use of switches SW-0 toSW-8. In one example similar to an example described above, zero (0) orother predetermined values may be stored in frequency bins associatedwith the blocked data, as well as the subcarrier corresponding to theblocked data. Further processing described above of such zeroes orpredetermined data by circuitry in DSP 1150 will result in null or zerodata outputs, for example, from a corresponding one of FEC decoders1260. Switch circuits provided at the outputs of FFTs 1210-1 and 1210-2,like switch circuits SW described above in FIG. 10b , may be provided toselectively insert zeroes or predetermined values for selectivelyblocking corresponding output data from DSP 1150. Such switches also maybe provided at the output of or within demultiplexers 1211-1 and 1211-2to selectively supply zero or predetermined values.

In another example, zeroes (0s) may be inserted in chromatic dispersionequalizer (CDEQ) circuits 1212 associated with both the X and Ypolarization components of each subcarrier. In particular, multipliercircuits (provided in corresponding butterfly filter circuits), likemultiplier circuits M described above, may selectively multiply theinputs to the CDEQ circuit 1212 by either zero or a desired coefficient.As discussed above in connection with FIG. 10c , multiplication by azero generates a zero product. When such zero products are furtherprocessed by corresponding circuitry in DSP 1150, e.g., correspondingIFFTs 1230, carrier phase correction components 1240, symbol-to-bitscomponents 1245, and FEC decoder, a corresponding output of DSP 1150will also be zero. Accordingly, data associated with a subcarrier SCreceived by a secondary node receiver 112, but not intended for outputfrom that receiver, can be blocked.

If, on the other hand, capacity requirements change and such previouslyblocked data is to be output from a given secondary node receiver DSP1150, appropriately coefficients may be supplied to the multipliercircuits, such that at least some of the inputs thereto are notmultiplied by zero. Upon further processing, as noted above, dataassociated with the inputs to the multiplier circuits and correspondingto a particular subcarrier SC is output from secondary node receiver DSP1150.

While FIG. 12 shows DSP 1150 as including a particular number andarrangement of functional components, in some implementations, DSP 650may include additional functional components, fewer functionalcomponents, different functional components, or differently arrangedfunctional components.

Upstream transmission from a secondary node 112 to primary node 110 willbe described next with reference to FIGS. 13a-13k and 14.

FIG. 13a shows an example of secondary node transmitter 304 in greaterdetail. Transmitter 304 includes a plurality of circuits or switches SW.In this example, nine switches (SW-0 to SW-8) are shown, although moreor fewer switches may be provided than that shown in FIG. 13a . Eachswitch may have, in one example, two inputs: the first input may receiveuser data, and the second input may receive control information orsignals (CNT). Each switch SW-0 to SW-8 further receives a respectiveone of control signals SWC-0 to SWC-8 output from a control circuit.Based on the received control signal, each switch SW-0 to SW8selectively outputs one of a respective one of data streams D-0 to D-8,a respective on of control signals CNT-0 to CNT-8, to DSP 1302.

Alternatively, in a manner similar to that noted above, the transmittedsubcarrier can be removed by inserting zero amplitude at the input oroutput of the FFT, or by programming zero coefficients into selectedmultiplier inputs of pulse shape filters 1008 and 1016, if a userdesires that particular subcarrier is to be omitted such that fewersubcarriers are to be transmitted.

DSP 1302 may have a similar structure as DSP 902 described above withreference to FIGS. 9 and 10 a. In some instances, however, DSP 1302 mayhave a lower capacity than DSP 902 and, therefore, the number ofcircuits, such as FEC encoders, bits-to-symbol mappers, overlap and savebuffers, FFT circuits, replicator circuits, and pulse shape filters maybe reduced, for example, in accordance with the number of inputs to DSP1302. Accordingly, fewer subcarriers may be output from each ofsecondary nodes 112 compared to the number of subcarriers output fromprimary node 110.

Based on the outputs of switches SW-0 to SW-8, DSP 1302 may supply aplurality of outputs to D/A and optics block 1301, which may have asimilar construction as D/A and optics block 901 described above tosupply X and Y polarized optical signals, each including I and Qcomponents, that are combined by a PBC and output onto an optical fibersegment 1316 included in one of optical communication paths 115 shown inFIG. 2.

Alternatively, based on zeroes (0s) stored or generated in DSP 1302,subcarriers may be blocked or added in a manner similar to thatdescribed above.

FIGS. 13b to 13e show examples of subcarriers (SC′) output from each ofsecondary nodes 112-j to 112-m, respectively, in an upstream directionwhen, for example, subcarriers SC0-SC19 shown in FIG. 6 are transmittedin the downstream direction. Namely, FIG. 13b shows a first group ofsubcarriers SC0′ to SC4′ which may be output from secondary node 112-j;FIG. 13c shows a second group of subcarriers SC5′ to SC9′ which may beoutput from secondary node 112-k; FIG. 13d shows a third group ofsubcarriers SC10′ to SC14′ which may be output from secondary node112-l; and FIG. 13e shows a fourth group of subcarriers SC15′ to SC19′which may be output from secondary node 112-m. As shown in FIG. 2, eachgroup subcarriers may be combined in combiner 116, for example andsupplied over optical communication path 117 to a receiver (e.g.,receiver 204 shown in FIG. 3) of primary node 110. FIG. 13f shows suchcombined optical subcarriers SC0′ to SC19′, each having a correspondingone of frequencies f0 to f19.

FIGS. 13g to 13j show example of subcarriers (SC″) output from each ofsecondary nodes 112-j to 112-m, respectively, in an upstream directionwhen, for example, subcarriers SC0-SC19 shown in FIG. 8 are transmittedin the downstream direction. In particular, FIG. 13g shows a first groupof subcarriers SC0″ to SC6″ and SC8″ which may be output from secondarynode 112-j; FIG. 13h shows a second group of subcarriers SC7″ andSC9″-SC11″ which may be output from secondary node 112-k; FIG. 13i showsa third group of subcarriers SC12″ to SC16″ which may be output fromsecondary node 112-l; and FIG. 13j shows a fourth group of subcarriersSC17″ to SC19″ which may be output from secondary node 112-m. As noted,each group subcarriers output from the secondary nodes may be combinedin combiner 116, for example and supplied over optical communicationpath 117 to receiver 204 in primary node 110. FIG. 13k shows suchcombined optical subcarriers SC0″ to SC19″, each having a correspondingone of frequencies f0 to f19.

FIG. 14 shows an example of primary node receiver 204 in greater detail.Primary node receiver 204 includes an Rx optics and ND block 1400 havinga similar construction as Rx optics and receiver block 1100 describedabove in connection with FIG. 11a . For example, block 1400 may includea polarization beam splitter that received an upstream polarizationmultiplexed optical signal including optical subcarriers SC0′ to SC19′or SC0″ to SC19″ supplied by optical fiber link 1401, which may be, forexample, an optical fiber segment as part of one of opticalcommunication paths 115-k to 115-m noted above, a local oscillatorlaser, optical hybrids, photodiodes, capacitive coupler, TIA/AGCcircuits and analog-to-digital (ND) converter circuits similar to thosediscussed above and configured in a manner similar to that describedabove. Accordingly, block 1400 provides outputs XI, XQ, YI, and YQsimilar to corresponding output supplied from block 1100. The bandwidthassociated with each of the components and circuits in block 1400,however, may have a greater bandwidth than the corresponding componentsin secondary nodes 112, and, therefore, primary node receiver 204 maydetect and process each of the subcarriers output from secondary nodes112.

As further shown in FIG. 14, primary node receiver 204 also may includeDSP 1450 that receives signals XI, XQ, YI, and YQ from block 1400. DSP1450 may have a similar structure as DSP 1150 shown in FIG. 11a . DSP1450, however, may have a higher capacity than DSP 1150 and, therefore,the number of circuits, such as FEC decoders, symbol to bit mappers,overlap and save buffers, CDEQ circuits, and PMDEQ circuits, may begreater than that shown in FIG. 11a in accordance with the number ofoutputs from DSP 1450.

The outputs from DSP 1450 next may be supplied to switches SW-O toSW-19, which selectively output, under the control of control signalssimilar to those described above, one of: a corresponding one of datastreams D-0 to D-19; and a corresponding one of control signals CNT-0 toCNT-19. The control information may include, for example, monitoringinformation associated with secondary nodes 112 or schedulinginformation, such as time of flight (TOF) or delta time TOF informationas described below with reference to FIGS. 20-24.

Alternatively, based on zeros generated or stored in DSP 1450, data maybe selectively be supplied from each DSP output in a manner similar tothat described above.

In a further example consistent with the present disclosure, guard bandsor frequency gaps may be provided between adjacent subcarriers SC. Asshown in FIG. 15, guard band GB1 may be provided between subcarriers SC4and SC5, and guard band GB2 may be provided between subcarriers SC5 andSC6. Additional guard bands may be provided between remaining adjacentpairs of subcarriers. Such guard bands may be provided in order todetect and process each subcarrier more accurately by reducing crosstalkor other interference between the subcarriers.

As noted above, network configurations other than the aggregationnetwork configuration discussed above in connection with FIGS. 1 and 2are contemplated here, such as a mesh network, an example of which(network 1800) is shown in FIG. 16. Network 1800 may include a pluralityof nodes 1802, 1804, 1806, 1808, 1810, and 1812, which are connected tohave multiple paths between one another. For example, node 1802 maycommunicate with node 1812 via a path including nodes 1806 and 1808 or apath including nodes 1804 and 1810. Nodes 1802, 1804, 1806, 1808, 1810,and 1812 may include at least one primary node, similar to node 110 andat least one secondary node 112 similar to those described above.Further, in a manner similar to that described above the amount of dataor data rate of data output from each of the nodes shown in FIG. 16 maybe controlled or dynamically adapted based on control informationexchanged between each node.

Allocation of data carried by subcarriers SC0 to SC19 and output fromsecondary nodes 112-j to 112-m, and examples of reallocation of suchdata output from the secondary nodes will next be described withreference to FIGS. 19 and 20 a to 20 c. FIG. 19 is a timing diagramshowing transmission of control information on dedicated subcarriersSC2, SC7, SC12, and SC17, as further depicted in FIG. 4. As shown in theexample depicted in FIG. 17, each of these subcarriers is continuouslytransmitted from primary node 110, while remaining subcarriers SC1,SC3-SC6, SC8-SC11, SC13-SC16, SC18, and SC19 are not transmitted. Thecontrol information carried by subcarriers SC2, SC7, SC12, and SC17 isassociated with nodes 112-j, 112-k, 112-l, and 112-m, respectively.Moreover, subcarriers SC2, SC7, SC12, and SC17 can also carry user dataassociated with each such node, in another example.

FIG. 18a shows a timing diagram corresponding to the subcarriertransmission shown in FIG. 6. Here, data associated with node 112-j iscontinuously transmitted on subcarrier SC0 to SC4, data associated withnode 112-k is continuously transmitted on subcarriers SC5 to SC9, dataassociated with node 112-l is continuously transmitted on subcarriersSC10 to SC14, and data associated with node 112-m is continuouslytransmitted on subcarriers SC15 to SC19.

As noted above, data associated with secondary nodes 112 may bereallocated among subcarriers SC0 to SC19. Consistent with presentdisclosure, such reallocation may occur on a time slot-by-time slotbasis. An example of such reallocation is shown in FIG. 18b . Namely, asshown in FIG. 18b , during time slot TS1 subcarrier SC8 carries dataassociated with node 112-k. During time slot TS2, however, subcarrier S8carries data associated with node 112-j and continues to do so untiltime slot TS6. At which point subcarrier S8 carries data associated withnode 112-k and does so through time slot S9, as well. In a similarfashion other time slots may be switched to carry data associated withdifferent nodes, or, as further shown in FIG. 18b , subcarriers, such assubcarriers SC0 to SC4 may continues to carry data associated with justnode 112-j. Such reallocation may be based on capacity requirements inthe system. In addition, as noted above, the data associated with aparticular secondary node 112 is that data which is output from thesecondary node based on control information received by the second nodeand the configuration of the second node, as described above withreference to FIGS. 10b and 10 c.

FIG. 18c shows a further example in which, in the absence of aparticular capacity need during a given time slot, no data istransmitted. Namely, as shown in FIG. 18c , during time slot TS5subcarriers SC11, SC8, and SC4 do not carry any. These subcarriers areblocked during time slot TS5 by circuitry described above with referenceto FIGS. 10b and 10 c.

The above examples describe reallocation of data to subcarrierstransmitted in the downstream direction from primary node 110 tosecondary nodes 112. In the upstream direction, as described above, oneor more subcarriers may be transmitted from one or more secondary nodes112 to primary node 110. If secondary nodes 112 are located at differentdistances away from primary node 110, the arrival time of data atprimary node 110 from one secondary node 112 may be different than thearrival time of data from another secondary node 112. As a result, datainput to the primary node 110 from secondary nodes 112 may not besynchronized, such that data launched on a given subcarrier from a firstsecondary node during one time slot may arrive while primary node 110 isreceiving data from another secondary node, leading to errors or loss ofdata at primary node 110.

For example, with reference to FIG. 19, if node 112-l is farther awayfrom primary node 110 than node 112-m, data output from node 112-l onsubcarrier SC4, for example, will experience a delay, td. Such data willnot arrive at primary node 110 until time t5+td, as shown in FIG. 19,and will continue to be received for the during of a time slot, i.e.,until time t6+td. At time t6, however, primary node 110 may being toreceive data from node 112-m, and due to the delay data transmissionfrom node 112-l, primary node 110 will, in this example, receive datafrom both node 112-l and 112-m on subcarrier 4, such that neither datasupplied from node 112-l nor data supplied from node 112-m may bereliably detected during the interval t6 to t6+td.

According to an aspect of the present disclosure, a method is providedby which circuitry in primary node 110 may determine the “time offlight” delay or the amount of time required for the primary node tosend a request to a secondary node and for the primary node to receive aresponse from the secondary node. Based on such time of flightinformation, primary node 110 may then schedule or coordinatetransmission on various time slots to avoid collisions, such as thatshown in FIG. 19.

FIG. 20 shows a flow chart of a method for determining time of flightinformation for each second node 112. In step 2002, circuitry in primarynode 110 initializes the time of flight of flight (TOF) offset for eachleaf 112 to zero. In step 2004, a test message or TOF offset is sent toeach leaf node 112 with an instruction or request for each such leafnode 12 to respond on a control channel, such as an in-band channel thatis transmitted with data and is carried by one of the subcarriers.Circuitry in primary node 110 next measures a response time for eachleaf node 112 (step 2006). Circuitry in primary node 110 next determinesthe difference in TOF times (“delta TOF”) for each node and each deltaTOF is adjusted so that the leaf node 112 having the longest TOF has adelta TOF of zero (step 2008). Next, circuitry in primary node 110determines when the delta TOF response time of each leaf node 112 iswithin a given tolerance or margin (step 2010). If not, steps 2004,2006, 2008, and 2010 are repeated and will continue to be repeated untilthe delta response time is within such margin. If the delta responsetime is within the margin, the above method starts again at step 2002 toreassess TOF information after a predetermined time period of Xmilliseconds.

Once the TOF and delta TOF information is obtained by the method shownin FIG. 20, primary node 110 schedules time slots to avoid collisionsthrough a further method having an associated flow chart shown in FIG.21. In a first step 2102 of the scheduling method, hub 110 evaluates ormeasures by way of the control channel noted above leaf node utilizationof subcarriers and time slots. Based on such measured utilization,circuitry in hub 110 determines whether a leaf node is within the TOFtolerance or margin (step 2104). If yes, one or more time slots areallocated to that leaf node (step 2106), and, if not, the hub nodeprovides instructions so that one or more subcarriers are allocated tosuch leaf node such that such leaf node may continuously transmit dataon such allocated subcarrier(s) to hub node 110. The leaf nodes are thenconfigured to transmit only during designated time slots or continuouslyover a one or more subcarriers in accordance with instructions received,for example, by way of the subcarriers noted above dedicated for sendingcontrol channel information (step 2110). In step 2112, the hub monitorleaf node utilization over a predetermined time period, such as a timeperiod of Y milliseconds. The process next returns to step 2102.

Note that the above TOF-related calculations may performed outside ofprimary node 110 or by circuitry inside primary node 110, as notedabove.

FIG. 22 is a plot showing examples of time hub request and leaf responsetimes, as well as corresponding TOF times. As depicted in FIG. 22, thefarther away a secondary node is from the primary node, the longer orthe greater the TOF time. For example, secondary node 112-m is located100 km away from primary node 110 and is thus the farthest secondarynode away from primary node 110. Further, in this example, secondarynode 112-l is 60 km away from primary node 110, and secondary node 112-kis 50 km away. In addition, node 112-j is located 30 km away fromprimary node 110 and is thus the closes secondary node to the primarynode.

TOF values are determined by circuitry in primary node 110 that measuresthe time it takes for the primary node to receive a response from asecondary node following a TOF request response (see steps 2004 and 2006above). As shown in FIGS. 22 and 23, the TOF associated with second node112-j is 300 μs (or 1.5 time slots (TS), assuming each time slot is 200μs); the TOF associated with node 112-k is 500 μs (or 2.5 TS); the TOFassociated with node 112-l is 600 μs (3.0 TS); and the TOF associatedwith node 112-m is 1000 μs (or 5.0 TS). As noted above, after each TOFis determined, circuitry in primary node 110, for example, determinesthe TOF associated with each node, wherein the delta TOF is thedifference between the longest TOF, here 1000 μs, and the TOF of eachremaining secondary node 112 (i.e., longest TOF—TOF of each remainingsecondary node). Accordingly, as further shown in table 2302, the deltaTOF associated with secondary node 112-j is 700 μs (1000 μs−300 μs=700μs or 3.5 TS); the delta TOF associated with secondary node 112-k is 500μs (1000 μs-500 μs=500 μs or 2.5 TS); the delta TOF associated withsecondary node 112-l is 400 μs (1000 μs−600 μs=400 μs or 2.0 TS); thedelta TOF associated with secondary node 112-m is 0.0 μs (1000 μs−1000μs=0.0 μs or 0 TS).

In scheduling the transmission from secondary node 112 to primary node110, both TOF and delta TOF are taken into account in order to achievedesired arrival times at primary node 110 for each such transmission. Anexample of such scheduling will next be described with reference to FIG.24.

FIG. 24 shows example timing charts 2402 and 2404 in which, for ease ofexplanation, transmission on only subcarriers SC1 to SC4 is shown. Chart2402 shows launch timing and leaf node utilization in absolute time forthe secondary nodes have the TOFs and delta TOFs noted in FIGS. 22 and23. Chart 2404 shows data carried by various subcarriers originating atsuch secondary nodes and corresponding arrival times at the primary nodewithout any collisions also in absolute time. In one example, as shownin FIG. 24, it is desirable that node 110 receive data on subcarriersSC1, SC2, and SC4 from secondary nodes 112-j, 112-k, and 112-mrespectively, at the beginning of time slot TS6. As noted above, sincesecondary node 112-m is the farthest from primary node 110, secondarynode 112-m has an associated delta TOF of zero (0). Data is thusscheduled to be transmitted, as shown in chart 2402, during time slotTS1, and, as shown in chart 2404, such data arrives five time slotslater during time slot TS6 (see FIGS. 22 and 23). Node 112-k, on theother hand, has a shorter time of flight of 2.5 TS. In order to arriveat node 110 at approximately the same time as the data output from node112-m, transmission from node 112-k is scheduled to be output orlaunched toward node 110 following the launch of the node 112-m data byan amount of time or delay D-k in chart 2402 approximately equal todelta TOF associated with node 112-k or 2.5 TS (see chart 2402). As aresult, following such delay, the node 112-k data is further delayed bythe TOF associated (2.5 TS) with node 112-k, such that the 112-k dataarrives at approximately the same time as the 112-m data (see chart2404).

Further, in order for data transmitted from node 112-j to arrive at node110 at approximately the same time as data output from node 112-m,transmission from node 112-j is scheduled to be launched toward node 110after the launch of the node 112-m data by an amount of delay D-japproximately equal to the delta TOF associated with node 112-j or 3.5TS (see chart 2402). Accordingly, following D-j, the node 112-k data isfurther delayed by the TOF associated (1.5 TS) with node 112-j, suchthat the 112-j data arrives at approximately the same time as the 112-mdata (see chart 2404).

Other example launch and arrival times and subcarrier utilization arefurther shown in FIG. 24.

In another example, empty time slots may be provided betweentransmission by one node on a given subcarrier and transmission byanother node on that subcarrier in order to provide sufficient timebetween slots so as to reduce the risk of collisions or datatransmission on a given subcarrier in which time slots overlap, forexample, as shown in FIG. 19.

Various modifications and other embodiments will be apparent to thoseskilled in the art from consideration of the present specification, andthe detailed implementations described above are provided as examples.For example, the digital signal process disclosed above may beimplemented as a programmable gate array circuit (PGA), or a fieldprogrammable gate array circuit (FPGA). In addition, although separatelasers 908 and 1110 are provided in the transmitter and receiver,respectively, as noted above, a transceiver consistent with the presentdisclosure may include a common laser that is “shared” between thetransmitter and receiver. For example, FIG. 25 is a diagram illustratingan example of the transceiver 110 using a shared laser 2502 providingoptical signals both for transmission and reception (as a localoscillator signal) in accordance with one or more implementations of thepresent disclosure. As shown, the laser 2502 generates an optical signaland provides the optical signal to the splitter 2504. The splitter 2504splits the optical signal into two portions. One portion is provided tothe optical hybrids or mixers 1120-1 and 1120-2, while the other portionis provided to modulators 910-1 to 910-4.

Accordingly, other implementations are within the scope of the claims.

What is claimed is:
 1. A receiver, comprising: a local oscillator laseroperable to supply light; optical hybrid circuits operable to mix thelight and optical subcarriers to provide mixing products; photodiodesoperable to convert the mixing products to electrical signals; analog todigital conversion circuitry operable to provide digital signals basedon the electrical signals; a processor operable to receive the digitalsignals and provide information indicative of data carried by at least agroup of the subcarriers based on the digital signals, wherein thereceiver is operable to selectively output the data.
 2. A receiver inaccordance with claim 1, wherein one of the plurality of subcarrierscarries control information associated with the receiver.
 3. A receiverin accordance with claim 2, wherein the control information identifiesan amount of data output from the receiver.
 4. A receiver in accordancewith claim 1, wherein the control information identifies a number ofsubcarriers carrying information indicative of data output from thereceiver.
 5. A receiver in accordance with claim 1, wherein each of theplurality of optical subcarriers is a Nyquist subcarrier.
 6. A receiverin accordance with claim 1, wherein each of the plurality of opticalsubcarriers does not spectrally overlap with another of the plurality ofoptical subcarriers.
 7. A receiver in accordance with claim 1, whereinthe processor is a digital signal processor.
 8. A receiver in accordancewith claim 1, wherein the processor has a plurality of outputs, thereceiver further including: switch circuitry coupled to at least one ofthe plurality of inputs.
 9. A receiver in accordance with claim 1,wherein the processor has a plurality of outputs, the transmitterfurther including: a plurality of switches, each of which is coupled toa respective one of the plurality of outputs.
 10. A receiver inaccordance with claim 9 wherein each of the plurality of switches isoperable to selectively supply one of a data or control information froma respective one of the plurality of outputs.
 11. A receiver inaccordance with 1, wherein a spectral gap is present between spectraassociated with at least first and second adjacent ones of the pluralityof optical subcarriers.
 12. A receiver in accordance with claim 1,wherein at least one of the plurality of optical subcarriers ismodulated in accordance with a modulation format, the modulation formatbeing one of an m-quadrature amplitude modulation (m-QAM) modulation,where m is a positive integer, a quadrature phase shift keying (QPSK)modulation format, or a binary phase shift keying (BPSK) modulationformat.